Apparatus and method for measuring time intervals with very high resolution

ABSTRACT

To measure relatively long time intervals with very high resolution, apparatus and method operate to receive a first pulse and a clock signal that has a known period, synchronize the first pulse with the clock signal, stretch the first synchronized pulse in accordance with a first stretch ratio, produce a first compared output pulse corresponding to the first stretched signal, synchronize the first compared output pulse with the clock signal, generate a first pulse sequence from the first synchronized pulse and the first synchronized compared output pulse, convert times of occurrences of the edges of the first pulse sequence to respective time values, receive a second pulse and generate a second pulse sequence in a manner similar to that of the first pulse sequence, convert times of occurrences of the edges of the second pulse sequence to respective time values, count the elapsed number of clock periods between the first and second synchronized pulses, derive the time interval between the received pulses from the time values, the first and second stretch ratios, the period of the clock and the elapsed number of clock periods, and calibrate the first and second stretch ratios from the time values.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/014,694, filed Apr. 2, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a technique for measuring timeintervals with very high resolution and, more particularly, to apparatusand method for measuring time intervals with very high resolution usinga time stretcher with a a time digitizer.

High resolution time measurements are used, for example, in particlephysics experiments to measure particle velocity. When the velocity andmomentum (measured by bending in a magnetic field) of a particle isknown, the mass of that particle can be calculated. The velocity in suchexperiments is calculated from a time of flight measurement over a knownflight path, wherein the time resolution should be better than theresolution of the particle detector, which generally has a maximumresolution of 100 picoseconds. Unfortunately, the time ranges in suchparticle physics experiments generally are less than 100 nanoseconds.

Other fields of instrumentation in which high resolution measurementsare taken include high speed digital sampling oscilloscopes, e.g., therandom interleaved sampling type, which may measure the time intervalbetween a trigger pulse and a sampling clock to a resolution that isbetter than the equivalent sampling period, which may be only a fewpicoseconds. Other instrumentation includes a laser ranging instrument(LIDAR) which often requires a resolution of less than 100 picosecondscorresponding to, for example, a distance of 16 mm, over a range ofseveral microseconds.

One known technique for measuring time intervals is to electronicallycount a clock down to resolutions of order 1 nanosecond, but theresolution is limited by the clock frequency and the ability tocorrectly count the clock. For a resolution of 1 nanosecond, a 1 GHzclock is required.

Another technique for measuring time intervals employs the digitalinterpolation of a slower clock, such technique achieving an RMSresolution that is better than 300 picoseconds using a 250 MHz clock. Afurther technique involves converting the time interval to a voltage orcharge and then measuring that voltage or charge with an analog todigital (ADC) converter. A time to amplitude converter (TAC) followed byan ADC suitably measures time in this manner.

Still another technique for measuring time intervals with highresolution is to use a time amplitude converter followed by a Wilkinsontype ADC, which converts the amplitude into a time and measures the timeby directly counting a clock with a scaler. By utilizing a timestretcher, the time interval to be measured is converted into aproportionally longer time interval and the longer time interval is thenmeasured, such measurement being reduced in scale by the amount of"stretch" of the time stretcher so as to produce the time intervalbetween two events.

In the above-discussed time measuring techniques, the maximum time rangethat can be measured is a function of the resolution of the ADC and thedesired time resolution. For example, given a 12 bit ADC and aresolution of 25 picoseconds, the maximum time interval that may bemeasured is only 100 nanoseconds.

One problem with the above-discussed time measuring techniques is thatsuch techniques and methods are "common start" or trigger methods inwhich the circuits are armed with a start signal, which enables the stopinput. While measuring the time interval between the start signal andstop input after it is armed is sufficient for many laboratorymeasurements in which the signals to be measured may be arranged asrequired, such common start measurement is unsuitable to satisfy thetime measuring requirements of time of flight measurements in particlephysics experiments. In such experiments, there are usually manyparticles passing through particle detectors and only selected particlesare to be measured. Since there are many extraneous signals, a triggerdecision is required, which involves providing a trigger signal afterthe event has occurred. This may be achieved by delaying the signal tobe measured by a relatively long amount of time so as to allow fortrigger formation and distribution, but such a signal delay requires arelatively long, high bandwidth and expensive delay line in each signalchannel.

Another problem with the above-discussed time measuring techniques isthat they record only a single hit. Since multiple hits on a channel arenot easily accommodated, the system (i.e., the time measuring device)must be reset after each measurement.

A further problem encountered in typical time measuring devices is thatthey are generally difficult to calibrate. Offset calibration and gaincalibration (e.g., the time stretching ratio) must be carried out foreach measuring channel, wherein the offset must always be calibrated bythe user since the relative signal paths from the signal sources to themeasuring instrument must be included in the offset. However, the scalefactor (i.e., the gain) is not easily measured and usually requirescreating test pulses with an accurately known variable time delay. Sincesuch calibration requires a special test setup, it is usually performedat a service bench. Further, calibration of the integral linearityrequires multiple measurements over the entire range of the instrument.Unfortunately, calibrating the time measuring device at a service benchrequires that it be quite stable over time and with respect to itsenvironment in order for the calibration to remain valid.

OBJECTS OF THE INVENTION

Therefore, it is an object of the present invention to provide apparatusand method for measuring time intervals with very high resolution whichovercome the shortcomings of the above-described devices.

Another object of the present invention is to provide a technique formeasuring time intervals with very high resolution in which both commonstart and common stop operation are possible.

A further object of the present invention is to provide a time intervalmeasuring device in which multiple measurements per channel are possiblewith modest dead time.

An additional object of this invention is to provide apparatus andmethod for measuring time intervals with very high resolution in whichthe time scale may be calibrated using normal data only.

Still another object of the present invention is to provide apparatusand method for measuring time intervals with very high resolution thatmay be completely calibrated in situ and wherein continuous calibrationusing the data itself can track slow changes in the scale factor thusobviating the need for a high stability design.

Various other objects, advantages and features of the present inventionwill become readily apparent to those of ordinary skill in the art, andthe novel features will be particularly pointed out in the appendedclaims.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, apparatusand method are provided for generating a difference pulse representing adifference between first and second received pulses, stretching thedifference pulse in accordance with a stretch ratio, producing acompared output pulse corresponding to the stretched signal, generatinga pulse sequence that has a first edge that corresponds to the firstpulse, a second edge that corresponds to the second pulse, and anotheredge that corresponds to the compared output pulse (i.e., the end of thestretched signal), converting times of occurrences of each of the edgesto respective time values, deriving the time interval between thereceived first and second pulses from the time values and the stretchratio, and calibrating the stretch ratio from the time values.

As one aspect of the present invention, the difference between thesecond and other time values is calculated and the differencetherebetween is divided by the stretch ratio to derive the time intervalbetween the first and second pulses.

As another aspect of the present invention, the difference between thesecond and other time values is calculated to produce the stretched timeinterval, the difference between the first and second time values iscalculated to produce the unstretched time interval, and the stretchratio is calibrated from the stretched time interval and the unstretchedtime interval.

As yet a further aspect of the present invention, the stretch ratio iscalibrated from the converted time values of a plurality of timeinterval measurements of a plurality of first and second pulses (e.g.,by averaging the stretch ratio of the measurements).

As yet another aspect of the present invention, a stop signal isreceived after the pulses are received, and a respective time differencebetween each edge of the pulse sequence and the stop signal isascertained so as to produce the respective time values.

In accordance with another embodiment of the present invention,apparatus and method are provided for receiving a first pulse and aclock signal having a predetermined known period, synchronizing thefirst pulse with the clock signal, stretching the first synchronizedpulse in accordance with a first stretch ratio, producing a firstcompared output pulse corresponding to the first stretched signal,synchronizing the first compared output pulse with the clock signal,generating a first pulse sequence from the first synchronized pulse andthe first synchronized compared output pulse, converting times ofoccurrences of each edge of the first pulse sequence to respective timevalues, receiving a second pulse, synchronizing the second pulse withthe clock signal, stretching the second synchronized pulse in accordancewith a second stretch ratio, producing a second compared output pulsecorresponding to the second stretched signal, synchronizing the secondcompared output pulse with the clock signal, generating a second pulsesequence from the second synchronized pulse and the second synchronizedcompared output pulse, converting times of occurrences of each edge ofthe second pulse sequence to respective time values, determining (e.g.,counting) the elapsed number of clock periods between the first andsecond synchronized pulses, deriving the time interval between thereceived first and second pulses from the time values, the first andsecond stretch ratios, the period of the clock and the elapsed number ofclock periods, and calibrating the first and second stretch ratios fromthe time values.

As one aspect of this embodiment of the present invention, the length intime of the first synchronized pulse is derived from the time values andthe first stretch ratio, the length in time of the second synchronizedpulse is derived from the time values and the second stretch ratio, theperiod of the clock is multiplied by the elapsed number of clock periodsto calculate the time interval between the first and second synchronizedpulses, and the time interval between the received first and secondpulses is derived from the lengths of the first and second synchronizedpulses and the time interval between the first and second synchronizedpulses.

As another aspect of this embodiment, the time intervals of the twostretched signals and the two unstretched signals are derived from thetime values, and the first and second stretch ratios are calibrated fromthe stretched and unstretched time intervals.

As yet a further aspect of this embodiment, the period of the clocksignal is accurately measured from some of the time values and thepredetermined known period.

As yet another aspect of this embodiment, the first and secondsynchronized pulses are each stretched by a predetermined number ofperiods of the clock signal, and the clock signal used to convert theedges of the pulse sequences to time values is the same clock signalused to synchronize the pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the present invention solely thereto, will best beappreciated in conjunction with the accompanying drawings, wherein likereference numerals denote like elements and parts, in which:

FIG. 1 is a block diagram of apparatus for measuring time intervals inaccordance with the present invention;

FIG. 2 is a schematic illustration of an exemplary circuit of the blockdiagram shown in FIG. 1 in accordance with the present invention;

FIGS. 3A-3F are various signal waveforms used to explain the operationof the time interval measuring apparatus of FIG. 1 in accordance withthe present invention;

FIG. 4 is a block diagram of a time interval measuring apparatus inaccordance with another embodiment of the present invention;

FIG. 5 is a schematic illustration of an exemplary circuit of the deviceof FIG. 4; and

FIGS. 6A-6E are various waveforms used for explaining the operation ofthe time interval measuring apparatus shown in FIG. 4 in accordance withthe present invention.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The time interval measuring apparatus and method of the presentinvention combines the functions of a standard time stretcher with thoseof a high-speed, pipelined, multiple hit, time digitizer to measure timeintervals with very high resolution. An exemplary time digitizer isLecroy's MTD133 monolithic integrated circuit which is an 8 channeldevice, fabricated in CMOS, and which has an LSB (least significant bit)of 500 picoseconds and a maximum measurement time range of 32microseconds. Each channel of the MTD133 can store up to 16 measurementswithin the 32 microsecond time range, and the double hit resolution (thedead time for a measurement to be recorded) is less than 10 nanoseconds.The exemplary MTD133 uses a tapped delay line to interpolate within the4 nanosecond period of its 250 MHz clock to achieve the 500 picosecondLSB, and the common reference input of the MTD133 can be either commonstart or common stop. The output data are the time differences betweenthe common reference and the signal inputs. It is appreciated thatalthough the present invention is described as utilizing a timedigitizer having the capabilities of the MTD133 integrated circuit,other suitable time digitizers may also be utilized.

As mentioned above and as will be seen from the following description,the present invention has many advantages over traditional highresolution timing methods. For example, both common start and commonstop operations are possible in the described embodiments, multiplemeasurements per channel are possible with only modest dead time,calibration of the time scale of the present invention is possiblewithout requiring special calibration signals, and a high stabilitydesign is not necessary to carry out the present invention, whereby theapparatus can be completely calibrated in situ and continuouscalibration using only the data itself can track changes in the scalefactor.

Referring now to the drawings, FIG. 1 is a block diagram of apparatusfor measuring time intervals with very high resolution in accordancewith a first embodiment of the present invention. For purposes ofconvenience, the time measuring device of FIG. 1 is referred to hereinas "simple time stretcher 10". An exemplary simple time stretcher isshown in FIG. 2 of the drawings. Simple time stretcher 10 measures thetime difference between a first event, which is a randomly occurringsignal pulse (a first pulse), such as shown in FIG. 3A of the drawings,and which is supplied from a detector (not shown), and a second event,which is a trigger pulse (a second pulse) (shown in FIG. 3B) whichindicates that an interesting event has occurred. Generally, the timedifference between the occurrence of the first pulse (i.e., thepositive-going edge of that pulse) and the occurrence of the second (ortrigger) pulse (i.e., the positive-going edge of that pulse) should begreater than some minimum value which is determined by the double pulseresolution of the time to digital converter being utilized (e.g., theMTD133).

This minimum value is approximately 10 nanoseconds for the MTD133 timedigitizer. Also, the widths of the two pulses both should be slightlylonger than the maximum time difference therebetween that is expected.

Referring to both FIGS. 1 and 2, the first pulse A and the trigger pulseB are supplied to a difference pulse generator 12 (FIG. 1), comprisedof, for example, gate 21 and transistor pair 22 in FIG. 2, whichgenerates therefrom a "difference" pulse, which is equal to, in Booleanform, (first pulse) AND (NOT (trigger pulse)), as shown in FIG. 3C. Asshown, the difference pulse begins at the leading edge of the firstpulse time and ends at the leading edge of the trigger pulse time.

The difference pulse signal is supplied to a capacitance circuit (i.e.,time stretcher 14) which includes therein a capacitor (e.g., capacitorC1 in FIG. 2) which is charged, for example, by integrating a constantcurrent source into the capacitor, during the existence of thedifference pulse. During this time, the capacitor voltage steadilyincreases to produce a ramp voltage, such as shown in FIG. 3D. At thetrailing edge of the difference pulse, the ramp voltage has peaked andis caused to decrease steadily thereafter by removing the accumulatedcharge in the capacitor with a relatively smaller current source of theopposite polarity with respect to the constant current source previouslysupplied to charge the capacitor. The ratio of the constant currentsource that charged the capacitor to the smaller current source thatdischarged the capacitor defines the time stretching ratio. Thecapacitor voltage, illustrated in FIG. 3D, is supplied from timestretcher 14 as the ramp voltage D.

The ramp voltage D is supplied to a comparator 16, e.g., comparator 24in FIG. 2, which turns on when the capacitor voltage is slightlydifferent from the capacitor's resting state voltage (V_(REST) or ref2)also supplied thereto so as to provide a high output when the capacitoris slightly charged, such as shown in FIG. 3E. As shown, the comparatorsupplies an output pulse (also referred herein as "pulse 4") whichextends from when the capacitor is initially charged to when thecapacitor is fully discharged.

The comparator's output (pulse 4), as well as the first pulse A and thetrigger pulse B, are supplied to a final pulse sequence generator 18,e.g., gates 26 and 28 in FIG. 2, which produces therefrom a "final"pulse sequence, such as shown in FIG. 3F. The generated final pulsesequence is equal to [(pulse 1) OR (pulse 4)] AND (NOT (pulse 2)). Thefinal pulse sequence F contains four transitions (edges) which aresupplied to a single channel of a time to digital converter 20, e.g.,the MTD133 time digitizer. As is clear from the above description, thefirst edge of the final pulse sequence occurs at the leading edge of thefirst pulse, the second edge occurs at the leading edge of the triggerpulse, the third edge occurs at the trailing edge of the trigger pulse,and the fourth edge occurs when the comparator turns off (i.e., thecapacitor is fully discharged). When the comparator turns off, and thecapacitor voltage has returned to its resting state, the simple timestretcher is ready for another pair of input pulses.

Time to digital converter 20, after receiving the four edges, receives acommon stop signal S_(STOP) which is derived from the trigger pulse andfurther trigger qualifications and which controls the simple timestretcher 10 to calculate the time difference between the first pulseand the trigger pulse from the four edges that are supplied to time todigital converter 20 (to be described). It is appreciated that the exacttime that common stop signal S_(STOP) is supplied to time to digitalconverter 20 is not important for purposes of the present invention. Aswill be seen from the ensuing description, each of the four edges of thefinal pulse sequence is measured with 500 picoseconds quantizing and aresolution that is better than 300 picoseconds RMS.

At the occurrence of the common stop signal S_(STOP) time to digitalconverter 20 ascertains the respective time difference between each ofthe edges of the final pulse sequence and the common stop signal toprovide four time difference times. The time difference between theoccurrences of the first pulse and the trigger pulse is ascertained withvery high resolution by calculating the difference between the times ofoccurrence of the second and fourth edges of the final pulse sequenceand dividing the calculated time difference by the stretch ratio. Aspreviously discussed, the stretch ratio is the ratio of the capacitor'scharging current source to its discharging current source.

As seen from FIGS. 3D and 3F of the drawings, the difference between thesecond and fourth edges of the final pulse sequence represents the"stretched time" and when this stretched time is divided by the stretchratio the precise time difference between the first and second edges ofthe final pulse sequence is ascertained. This time difference also isequal to the time difference between the first and trigger pulses. Witha stretch ratio of, for example, 20:1 and 500 picosecond quantizing, thefinal least significant bit (LSB) is 25 picoseconds.

In accordance with the present invention, the stretch ratio of thecapacitor is precisely calibrated using only data that is derived fromsample pulse signals (i.e., not special calibration signals) supplied tosimple time stretcher 10. If the maximum time difference expected is 100nanoseconds, the stretch ratio must be known to be better than one partin four thousand. The stretch ratio can be calculated from the data byusing the first edge time measurement, and the difference between thefirst and second edges of the final pulse sequence measured with 500picosecond LSB.

From the time difference between the first and second edges of the finalpulse sequence, the stretch ratio is calculated with an RMS error noworse than one part in thirty, which is the error if the time differenceis the minimum allowed, e.g., 10 nanoseconds. The average RMS error isabout one part in 150, and by averaging the stretch ratio using manymeasurements, the error can be reduced to an arbitrarily small value.

LeCroy's MTD133 time digitizer, which may be utilized as time to digitalconverter 20 in the present invention, mathematically reduces the errordiscussed above by averaging a sufficient number of measurements, butthree conditions should be satisfied for the MTD133 to accomplish such.First, the times measured by the MTD133 should be random with respect toits 250 MHz clock, and there should be no correlation between the clockand the input signals. Second, the 250 MHz clock should be stable andits period should be precisely known, and the phase noise of the clockshould be very small. Third, the differential non-linearity due to theinterpolation of the clock period should be symmetric, and this occursautomatically since the time measurements of interest are alldifferences between times measured with the same measuring channel. Thedifferential non-linearity is not canceled by the subtraction, but isforced to be symmetric which means that the distribution of errors willbe symmetric and the mean of the error distribution will converge to thecorrect value for a large number of measurements.

The RMS error in the calculated stretch ratio is reduced approximatelyas one over the square root of N, where N is the number of measurements.To reduce the average error to one part in 4,000, the square root of Nshould be at least 27, thus requiring about 700 measurements. Thesemeasurements should be of randomly distributed time differences over thefull time range expected. It is appreciated that, in accordance with thepresent invention, the above operations and calibration measurements areobtained using "ordinary" data that has been gathered during anexperiment. Furthermore, if the measurements of actual time and stretchtime are fit to a straight line, rather than simply averaging thestretch ratio, non-linearities can be detected. If the system is notlinear, then the stretch ratio is different for small and large timedifferences, and this non-linearity can be accommodated by adjusting thefitting function, and the number of measurements required to accommodatethis non-linearity depends on the nature and extent of thatnon-linearity.

As is appreciated, the simple time stretcher of the present inventionprovides good protection against data corruption by randomly occurringunrelated signals. That is, the pattern of the four edges of the finalpulse sequence can be used to verify that the measurements are notcorrupted. Namely, the difference between the first and second edgesshould be within the minimum and maximum values allowed, the differencebetween the second and third edges should equal the width of the triggerpulse, and the calculated stretch ratio for each individual event shouldbe consistent with the calibrated value.

Examples of instances when a measurement will be corrupted include anevent in which the trigger pulse does not overlap the first pulse butoverlaps the comparator output (here, the consistency test will fail),and an event with multiple input signals during the stretching time.Further, a random event that occurs either well before or just after thetrigger will provide only one pulse whose width is equal to the firstpulse width plus that width times the stretch ratio. This pulse (width)is the system dead time for non-triggered input signals. For example,for a system having a 100 nanosecond maximum time difference and 20:1stretch ratio, the maximum dead time is approximately 2.1 microseconds.

In accordance with another embodiment of the present invention,hereinafter referred to as a "long time base stretcher", the timedifference between two events that are separated by more than 100nanoseconds and less than 32 microseconds is measured. FIG. 4 is afunctional block diagram of the long time base stretcher of the presentinvention and FIG. 5 is an exemplary circuit of the long time basestretcher, wherein a separate clock oscillator (shown in FIG. 5)supplies a clock signal CLK with a period near 10 nanoseconds, but notexactly 10 nanoseconds. It is appreciated, however, that other clockperiods may be utilized. The clock should not be commensurate with the250 MHz clock which drives time to the digital converter 40 and, in apreferred embodiment of the present invention, this clock signal has aperiod of 10.005 nanoseconds and has a phase slip of 5 picoseconds everycycle with respect to the 250 MHz clock. This clock period is sufficientto calibrate a 25 picosecond LSB, even if the clocks beat with a precise5 picosecond phase slip. Clock signal CLK is illustrated in FIG. 6A ofthe drawings.

A first pulse, which represents the first of two events in which thetime difference therebetween is measured and which is shown in FIG. 6B,is supplied to a pulse synchronizing circuit 30 which includes, forexample, D flip flops 42-44, gates 46 and transistor pair 48 of FIG. 5.The pulse synchronizing circuit synchronizes the first pulse with clockCLK to produce a pulse which has a leading edge at the leading edge ofthe first pulse and a trailing edge at the second leading clock edgeafter the first pulse, as shown in FIG. 6C. As seen from FIGS. 6A-6C,the trailing edge of the synchronized pulse is "synchronized" with clockCLK. Since clock CLK has a period of 10.005 nanoseconds, thesynchronized pulse varies in width from 10.05 to 20.1 nanoseconds,depending on the phase of the clock at the arrival of the first pulse.

The synchronized pulse is supplied to a time stretcher circuit 32, e.g.,capacitor 50 (FIG. 5), and similar to the time stretcher circuit of FIG.1 (i.e., time stretcher 14), time stretcher circuit 32 produces the rampsignal D shown in FIG. 6D and supplies the ramp signal D to a comparator34 (comparator 52 in FIG. 5) which produces a comparator output signalsimilar to the comparator output of comparator 16 of FIG. 1 (see FIG.3E). The comparator output is supplied to a comparator outputsynchronizer 36 (gate 54) which synchronizes the trailing edge of thecomparator output with clock CLK to produce a second synchronized pulse,such as shown as the second pulse of FIG. 6E. The second synchronizedpulse is supplied along with the first synchronized pulse (FIG. 6C) to afinal pulse sequence generator 38 which combines the two pulses in an ORcircuit, e.g., gate 56, to produce the pulse waveform shown in FIG. 6E.The four edges of the two pulses are supplied to time to digitalconverter 40.

As previously discussed, the output of comparator 34 is synchronized incircuit 36 with clock CLK. In an alternative embodiment of the presentinvention, the output of comparator 34 is not synchronized with clockCLK, and instead, a simple monostable is sufficient since the fourthedge of the pulse sequence shown in FIG. 6E is not utilized to derivethe time difference between the first two edges of the pulse sequence(to be discussed). However, in a preferred embodiment of the presentinvention, the output of comparator 34 is synchronized to clock CLK insynchronizer circuit 36 to provide the advantageous result of preciselymeasuring the clock period of clock CLK using the times of the secondand fourth edges of the pulse sequence, both of which are synchronizedwith clock CLK. As will be described, the clock period itself isutilized to derive the ultimate time difference between the first andtrigger pulses. Furthermore, synchronizing the output of comparator 34with clock CLK allows for a simple reset mechanism which, inter alia,can enforce a short recovery time before allowing a new first pulse torestart the measuring cycle.

Time to digital converter 40 (e.g., MTD133) converts the four edges ofthe pulse sequence shown in FIG. 6E to time values in a manner similarto that described previously with respect to the simple time stretcher.In the long time base stretcher, however, the time difference betweenthe first and second edges of the pulse sequence of FIG. 6E is derivedby calculating the time difference between the second and third edges(the "stretched" time) and dividing that time difference by the stretchratio. The stretch ratio is calibrated for each channel in thetime-to-digital converter using the same method described above in thesimple time stretcher, and also as in the simple time stretcher case, nospecial calibration data is required. Furthermore, the input time andthe phase of clock CLK should be random with respect to the 250 MHzclock of the time-to-digital converter. However, the long time basestretcher's stretch ratio needs to be known only to about 1 part in 1000and since the average error for a single measurement is 1 part in 50,only four hundred random measurements are required to calibrate achannel.

The total dead time for a long time base stretcher that has a 10nanosecond separate clock period CLK and a 20:1 stretch ratio is 450nanoseconds maximum and 350 nanoseconds average. This dead timedetermines the multiple hit capability of the channel and unlike thesimple time stretcher, the long time base stretcher is adapted so thateach measurement cannot be corrupted by locking out other signals duringthe stretch time until the circuits are reset.

A second (trigger) pulse is measured the same way the first pulse ismeasured, possibly on a different long time base stretcher circuit and adifferent channel of time-to-digital converter 40. As previouslydiscussed, the long time base stretcher measures the time differencebetween two events which are separated by more than 100 nanoseconds andless than 32 microseconds. The time difference between the first twoedges of the pulse sequence shown in FIG. 6E is determined by a firsttime base stretcher circuit in the manner discussed above. Similarly,the time difference between the first two edges of a pulse sequenceproduced in a second time base stretcher is separately calculated. Thetiming diagrams of the second pulse, the synchronized pulse, the rampsignal and pulse sequence of the second time base stretcher are similarto the timing diagrams of FIGS. 6B-6E, respectively.

In the long time base stretcher, the same clock signal CLK is suppliedto all of the time stretcher channels and the number of clock cycles ofclock signal CLK that occur from the occurrence of the synchronizingedge time of the first pulse (the trailing edge of the synchronizedpulse shown in FIG. 6C) and the synchronizing edge time of the second(trigger) pulse (similarly, the trailing edge of the synchronized pulsein the second time base stretcher) is counted.

Finally, the time difference between the occurrences of the first pulseand the second (trigger) pulse is calculated by subtracting the timemeasured by the second time stretcher (i.e., the time difference betweenthe first two edges of the pulse sequence produced from the secondpulse) from the time measured by the first time stretcher (i.e., thetime difference between the first two edges of the pulse sequence shownin FIG. 6E), and adding the amount of time that has elapsed between thesynchronizing edges of the respective time base stretchers, which isequal to the number of clock cycles counted multiplied by the preciselyknown clock period of clock CLK. Since the second and fourth edges ofeach pulse sequence are synchronized with clock CLK, and time-to-digitalconverter 40 converts those edges into time values, the period of clockCLK is accurately measured by averaging many measurments.

The stretch ratio is calibrated for each channel in the mannerpreviously described. The LSB of each stretcher circuit will bedifferent and none will be exactly 25 picoseconds, and the stretch ratioof each circuit also will be different. At the end points of thestretcher range, it is likely that there will be a "bin" with a muchsmaller width causing a local differential non-linearity. Since thefinal number is a difference of two measurements and the phase of clockCLK is random with respect to the pulses being measured, these errorssimply become long tails on the error distribution and increase thefinal RMS error slightly.

Therefore, the final time difference between two events can be measuredwith an LSB of approximately 25 picoseconds over a range of up to 32microseconds. In a preferred embodiment, clock CLK should have anaccuracy of better than 1 part in two million since the LSB of the timebase stretcher is limited by the accuracy and stability of this clock.

In another embodiment of the present invention, the time differencebetween two events is measured with very high resolution in a "very highresolution time stretcher" circuit. The very high resolution timestretcher is similar to the long time base stretcher circuit shown inblock diagram form in FIG. 4, except clock signal CLK is not utilized,and instead, the input pulses (i.e., the first pulse and the second(trigger) pulse) are synchronized with the 250 MHz clock of the MTD133(i.e., the time to digital converter). The input pulses are synchronizedwith the 250 MHz clock in a manner similar to that described above so asto produce respective synchronized pulses which are 4 to 8 nanosecondslong. FIG. 6C of the drawings illustrates the synchronized pulse whenthe clock signal of FIG. 6A is the 250 MHz clock (a 4 nanosecondperiod). The synchronized pulse (for both the first and trigger pulses)is stretched in the manner described above and recorded in the time todigital converter. That is, the four edges of the pulse sequence, suchas shown in FIG. 6E, are converted to time values in the time to digitalconverter. This is quite similar to the operation of the long time basestretcher discussed above, except that the synchronized pulse (i.e., thefirst pulse of the pulse sequence) has a minimum width of only 4nanoseconds which is too short for the time to digital converter tocorrectly record both edges thereof. In accordance with the presentinvention, the 4 to 8 nanosecond long synchronized pulse is supplied tothe time stretcher circuit, but that pulse is lengthened by two clockperiods (i.e., 8 nanoseconds) before it is supplied to the time todigital converter. Thus, the first of the two pulses in the pulsesequence that is supplied to the time to digital converter is 12 to 16nanoseconds long, which is long enough for the time to digital converterto correctly record both of its edges. In other words, the pulsesequence supplied to the time to digital converter is similar to thepulse sequence shown in FIG. 6E, except the first pulse (whichrepresents the time of the synchronized pulse (FIG. 6C)) is extended byan additional two clock pulses.

For a 20 to 1 stretch ratio in the very high resolution time stretcher,the dead time is 180 nanoseconds maximum, which is considerably lessthan the maximum dead time of 450 nanoseconds in the long time basestretcher. Also, the stretch ratio calibration of the very highresolution time stretcher can be less precise than that of the long timebase stretcher, which translates into improved multiple hit capabilityand quicker calibration. On the other hand, the stretch ratio can beincreased to reduce the final LSB of the time stretcher circuit'soutput. It is appreciated that the elimination of a separate clockremoves any possible beating effects which produce subtlenon-linearities. Thus, for a stretch ratio of 500 to 1, the final LSB isone picosecond, and since the interpolation is over only 4 nanoseconds,the stretch ratio needs to be calibrated to one part in 4000, which isthe same accuracy that is required in the simple time stretcher circuit.

Although the calibration procedure for the very high resolution timestretcher circuit is slightly more complicated than for the othercircuits discussed above, such calibration can be accomplished withoutany special calibration inputs. Also, since the trailing edge of thesynchronized pulse has a fixed phase relationship with the clock, thestretch ratio cannot be measured precisely by averaging manymeasurements on a single channel. However, calibrating two or morechannels simultaneously is possible because the leading edges of bothinput pulses are random with respect to the clock. Such requires fittingfor the scale factor for each channel and the relative offsetsimultaneously, and instead of fitting the data to a two dimensionalcurve for a single channel, the fit is to a three dimensional surface.And this is readily generalized to calibrate many channelssimultaneously.

While the present invention has been particularly shown and described inconjunction with preferred embodiments thereof, it will be readilyappreciated by those of ordinary skill in the art that various changesmay be made without departing from the spirit and scope of theinvention.

Therefore, it is intended that the appended claims be interpreted asincluding the embodiments described herein, the alternatives mentionedabove, and all equivalents thereto.

What is claimed is:
 1. Apparatus for measuring a time interval betweenoccurrences of first and second pulses, comprising:means for receivingfirst and second pulses; difference pulse generating means forgenerating a difference pulse representing a difference between saidfirst and second pulses; time stretching means for stretching saiddifference pulse in accordance with a stretch ratio to produce astretched signal; means for producing a compared output pulsecorresponding to said stretched signal; pulse sequence generator meansfor generating a pulse sequence having a plurality of edges; a first ofsaid edges corresponding to said first pulse, a second of said edgescorresponding to said second pulse, and another one of said edgescorresponding to said compared output pulse; time to digital convertermeans for converting times of occurrences of said first edge, saidsecond edge and said another edge of said pulse sequence to respectivefirst, second, and another time values, and for deriving said timeinterval between the received first and second pulses from said secondtime value, said another time value and said stretch ratio; andcalibration means for calibrating the stretch ratio of said timestretching means from said first, second and another time values.
 2. Theapparatus of claim 1, wherein said time to digital converter meansincludes means for calculating a difference between said second and saidanother time values, and for dividing the determined difference by saidstretch ratio to derive said time interval between the received firstand second pulses.
 3. The apparatus of claim 1, wherein said time todigital converter means includes means for calculating a differencebetween said second and said another time values to produce a stretchedtime interval representing a time length of said stretched signal; andsaid calibration means includes means for calculating a differencebetween said first and second time values to produce an unstretched timeinterval representing a time interval between said first and secondpulses before stretching; and means for calibrating the stretch ratiofrom said stretched time interval and said unstretched time interval. 4.The apparatus of claim 1, wherein said calibration means calibrates thestretch ratio of said time stretching means from said first, second andanother time values of a plurality of time interval measurements of aplurality of first and second pulses.
 5. The apparatus of claim 4,wherein said calibration means includes means for averaging the stretchratio of a plurality of time interval measurements to produce acalibrated stretch ratio.
 6. the apparatus of claim 1, wherein saiddifference pulse generating means includes means for inverting saidsecond pulse to produce an inverted second pulse; and means forlogically multiplying said first pulse and the inverted second pulse toproduce said difference pulse.
 7. The apparatus of claim 1, wherein saidmeans for producing a compared output pulse includes means for comparingsaid stretched signal to a predetermined resting value and for producingsaid compared output signal corresponding to when said stretched signalis greater than said predetermined resting value.
 8. The apparatus ofclaim 1, wherein said first edge of said pulse sequence corresponds to atime of occurrence of a leading edge of said first pulse, said secondedge of said pulse sequence corresponds to a time of occurrence of aleading edge of said second pulse, and said another one of said edges ofsaid pulse sequence corresponds to a trailing edge of said comparedoutput pulse.
 9. The apparatus of claim 1, wherein said time to digitalconverter means includes means for receiving a stop signal after thepulse sequence is generated; and means for ascertaining respective timedifferences between said first edge, said second edge and said anotheredge of said pulse sequence and said stop signal to produce said first,second and another time values.
 10. Apparatus for measuring a timeinterval between occurrences of first and second pulses,comprising:means for receiving a first pulse, a second pulse and a clocksignal, said clock signal having a predetermined known period;synchronizing means for synchronizing the first received pulse with saidclock signal to produce a first synchronized pulse, and forsynchronizing the second received pulse with said clock signal toproduce a second synchronized pulse; means for determining an elapsednumber of clock periods between occurrences of the first synchronizedpulse and the second synchronized pulse; time stretching means forstretching said first synchronized pulse in accordance with a firststretch ratio to produce a first stretched signal, and for stretchingsaid second synchronized pulse in accordance with a second stretch ratioto produce a second stretched signal; compared output means forproducing a first compared output: pulse corresponding to said firststretched signal, and for producing a second compared output pulsecorresponding to said second stretched signal; compared outputsynchronizing means for synchronizing the first compared output pulsewith said clock signal to produce a first synchronized compared outputpulse, and for synchronizing the second compared output pulse with saidclock signal to produce a second synchronized compared output pulse;pulse sequence generator means for generating a first pulse sequencefrom the first synchronized pulse and the first synchronized comparedoutput pulse, and for generating a second pulse sequence from the secondsynchronized pulse and the second synchronized compared output pulse;time to digital converter means for converting times of occurrences ofeach of said edges of said first pulse sequence to respective timevalues; for converting times of occurrences of each of said edges ofsaid second pulse sequence to other respective time values; and forderiving said time interval between the received first and second pulsesfrom said time values and from said other time values, the first andsecond stretch ratios, the period of the clock and the elapsed number ofclock periods; and calibration means for calibrating the first stretchratio from said time values, and for calibrating the second stretchratio from said other time values.
 11. The apparatus of claim 10,wherein said means for receiving receives said first pulse on a firstchannel of said apparatus and receives said second pulse on a secondchannel of said apparatus; and each of said means of said apparatusutilizes the same received clock signal.
 12. The apparatus of claim 10,wherein said time to digital converter means converts times ofoccurrences of first, second, third and fourth edges of said first pulsesequence to first, second, third and fourth time values, respectively;and converts times of occurrences of first, second, third and fourthedges of said second pulse sequence to fifth, sixth, seventh and eighthtime values, respectively.
 13. The apparatus of claim 12, wherein saidtime to digital converter means is operable to derive said time intervalbetween the received first and second pulses from the second, third,sixth and seventh time values, the first and second stretch ratios, theperiod of the clock and the elapsed number of clock periods.
 14. Theapparatus of claim 12, wherein said calibration means calibrates thefirst stretch ratio from said first, second and third time values, andcalibrates the second stretch ratio from said fifth, sixth and seventhtime values.
 15. The apparatus of claim 12, wherein said time to digitalconverter means includes calculating means for deriving a length in timeof said first synchronized pulse from said second time value, said thirdtime value and said first stretch ratio; for deriving a length in timeof said second synchronized pulse from said sixth time value, saidseventh time value and said second stretch ratio; for multiplying theperiod of the clock and the elapsed number of clock periods to produce atime interval between a synchronized edge of said first synchronizedpulse and a synchronized edge of said second synchronized pulse; and forderiving said time interval between the received first and second pulsesfrom the lengths in time of said first and second synchronized pulses,and said time interval between the synchronized edges of said first andsecond synchronized pulses.
 16. The apparatus of claim 15, wherein saidcalculation means of said time to digital converter means is operable tocalculate a difference between said second and third time values, and todivide the determined difference by said first stretch ratio to derivesaid length in time of said first synchronized pulse; is operable tocalculate a difference between said sixth and seventh time values, andto divide this determined difference by said second stretch ratio toderive said length in time of said second synchronized pulse; and toderive said time interval between the received first and second pulsesby subtracting the length in time of said second synchronized pulse fromthe length in time of said first synchronized pulse and adding theretosaid time interval between the synchronized edges of said first andsecond synchronized pulses.
 17. The apparatus of claim 12, wherein saidsynchronizing means produces said first synchronized pulse having aleading edge corresponding to the first received pulse and a trailingedge synchronized with an edge of one pulse of said clock signal; andproduces said second synchronized pulse having a leading edgecorresponding to the second received pulse and a trailing edgesynchronized with an edge of another pulse of said clock signal.
 18. Theapparatus of claim 17, wherein said time to digital converter meansincludes means for calculating a difference between said second andthird time values to produce a first stretched time intervalrepresenting a time length of said first stretched signal; and forcalculating a difference between said sixth and seventh time values toproduce a second stretched time interval representing a time length ofsaid second stretched signal; and said calibration means includes meansfor calculating a difference between said first and second time valuesto produce a first unstretched time interval representing a length intime of said first synchronized pulse before stretching; for calculatinga difference between said fifth and sixth time values to produce asecond unstretched time interval representing a length in time of saidsecond synchronized pulse before stretching; and means for calibratingthe first stretch ratio from said first stretched time interval and saidfirst unstretched time interval, and for calibrating the second stretchratio from said second stretched time interval and said secondunstretched time interval.
 19. The apparatus of claim 17, wherein saidmeans for determining counts said elapsed number of clock periodsbetween the synchronized edge of said one pulse and the synchronizededge of said another pulse of said clock signal.
 20. The apparatus ofclaim 17, further comprising means for accurately measuring the numberof periods of said clock signal from said second, fourth, sixth andeighth time values and said predetermined known period.
 21. Theapparatus of claim 10, wherein said calibration means calibrates thefirst stretch ratio and the second stretch ratio from time values of aplurality of time interval measurements of a plurality of first andsecond pulses.
 22. The apparatus of claim 10, wherein said time todigital converter means generates each of said time values in accordancewith a clock signal, and is further operable to supply said clock signalto said means for receiving.
 23. The apparatus of claim 10, wherein saidpulse sequence generator means includes a first summing circuit forcombining the first synchronized pulse and the first synchronizedcompared output pulse to produce the first pulse sequence, and a secondsumming circuit for combining the second synchronized pulse and thesecond synchronized compared output pulse to produce the second pulsesequence.
 24. The apparatus of claim 10, wherein said time to digitalconverter means includes means for receiving a stop signal; means forascertaining respective time differences between first, second, thirdand fourth edges of said first pulse sequence and said stop signal toproduce first, second, third and fourth time values; and means forascertaining respective time differences between first, second, thirdand fourth edges of said second pulse sequence and said stop signal toproduce fifth, sixth, seventh and eighth time values.
 25. Method ofmeasuring a time interval between occurrences of first and secondpulses, comprising the steps of:receiving first and second pulses;generating a difference pulse representing a difference between saidfirst and second pulses; stretching said difference pulse in accordancewith a stretch ratio to produce a stretched signal; producing a comparedoutput pulse corresponding to said stretched signal; generating a pulsesequence having a plurality of edges; a first of said edgescorresponding to said first pulse, a second of said edges correspondingto said second pulse, and another one of said edges corresponding tosaid compared output pulse; converting times of occurrences of saidfirst edge, said second edge and said another edge of said pulsesequence to respective first, second, and another time values; derivingsaid time interval between the received first and second pulses fromsaid second time value, said another time value and said stretch ratio;and calibrating the stretch ratio from said first, second and anothertime values.
 26. The method of claim 25, wherein said deriving step iscarried by calculating a difference between said second and said anothertime values, and dividing the determined difference by said stretchratio to derive said time interval between the received first and secondpulses.
 27. The method of claim 25, wherein said deriving step includesthe step of calculating a difference between said second and saidanother time values to produce a stretched time interval representing atime length of said stretched signal; and said calibration step iscarried out by calculating a difference between said first and secondtime values to produce an unstretched time interval representing a timeinterval between said first and second pulses before stretching, andcalibrating the stretch ratio from said stretched time interval and saidunstretched time interval.
 28. The method of claim 25, wherein saidcalibration step is carried out by calibrating the stretch ratio fromsaid first, second and another time values of a plurality of timeinterval measurements of a plurality of first and second pulses.
 29. Themethod of claim 28, wherein said calibration step is carried out byaveraging the stretch ratio of a plurality of time interval measurementsto produce a calibrated stretch ratio.
 30. The method of claim 25,wherein said step of generating a difference pulse is carried out byinverting said second pulse to produce an inverted second pulse, andlogically multiplying said first pulse and the inverted second pulse toproduce said difference pulse.
 31. The method of claim 25, wherein saidstep of producing a compared output pulse is carried out by comparingsaid stretched signal to a predetermined resting value and producingsaid compared output signal corresponding to when said stretched signalis greater than said predetermined resting value.
 32. The method ofclaim 25, wherein said first edge of said pulse sequence corresponds toa time of occurrence of a leading edge of said first pulse, said secondedge of said pulse sequence corresponds to a time of occurrence of aleading edge of said second pulse, and said another one of said edges ofsaid pulse sequence corresponds to a trailing edge of said comparedoutput pulse.
 33. The method of claim 25, wherein said converting stepincludes the steps of receiving a stop signal after the pulse sequenceis generated, and ascertaining respective time differences between saidfirst edge, said second edge and said another edge of said pulsesequence and said stop signal to produce said first, second and anothertime values.
 34. Method of measuring a time interval between occurrencesof first and second pulses, comprising the steps of;receiving a firstpulse and a clock signal, said clock signal having a predetermined knownperiod; synchronizing the first received pulse with said clock signal toproduce a first synchronized pulse; stretching said first synchronizedpulse in accordance with a first stretch ratio to produce a firststretched signal; producing a first compared output pulse correspondingto said first stretched signal; synchronizing the first compared outputpulse with said clock signal to produce a first synchronized comparedoutput pulse; generating a first pulse sequence from the firstsynchronized pulse and the first synchronized compared output pulse;converting times of occurrences of each of said edges of said firstpulse sequence to respective time values; receiving a second pulse;synchronizing the second received pulse with said clock signal toproduce a second synchronized pulse; stretching said second synchronizedpulse in accordance with a second stretch ratio to produce a secondstretched signal; producing a second compared output pulse correspondingto said second stretched signal; synchronizing the second comparedoutput pulse with said clock signal to produce a second synchronizedcompared output pulse; generating a second pulse sequence from thesecond synchronized pulse and the second synchronized compared outputpulse; converting times of occurrences of each of said edges of saidsecond pulse sequence to other respective time values; determining anelapsed number of clock periods between occurrences of synchronizededges of the first synchronized pulse and the second synchronized pulse;deriving said time interval between the received first and second pulsesfrom the time values and the other time values, the first and secondstretch ratios, the period of the clock and the elapsed number of clockperiods; calibrating the first stretch ratio from said time values; andcalibrating the second stretch ratio from said other time values. 35.The method of claim 34, wherein said first receiving step receives saidfirst pulse on a first channel; said second receiving step receives saidsecond pulse on a second channel; and each of said steps is carried outusing the same received clock signal.
 36. The method of claim 34,wherein said first converting step is carried out by converting times ofoccurrences of first, second, third and fourth edges of said first pulsesequence to first, second, third and fourth time values, respectively;and said second converting step is carried out by converting times ofoccurrences of first, second, third and fourth edges of said secondpulse sequence to fifth, sixth, seventh and eighth time values,respectively.
 37. The method of claim 36, wherein said step of derivingis carried out by deriving said time interval between the received firstand second pulses from the second, third, sixth and seventh time values,the first and second stretch ratios, the period of the clock and theelapsed number of clock periods between the synchronized edges of thefirst and second synchronized pulses.
 38. The method of claim 36,wherein said first calibration step is carried out by calibrating thefirst stretch ratio from said first, second and third time values; andsaid second calibration step is carried out by calibrating the secondstretch ratio from said fifth, sixth and seventh time values.
 39. Themethod of claim 36, wherein said step of deriving said time intervalincludes the steps of deriving a length in time of said firstsynchronized pulse from said second time value, said third time valueand said first stretch ratio; deriving a length in time of said secondsynchronized pulse from said sixth time value, said seventh time valueand said second stretch ratio; multiplying the period of the clock andthe elapsed number of clock periods to produce a time interval between asynchronized edge of said first synchronized pulse and a synchronizededge of said second synchronized pulse; and deriving said time intervalbetween the received first and second pulses from the lengths in time ofsaid first and second synchronized pulses, and said time intervalbetween the synchronized edges of said first and second synchronizedpulses.
 40. The method of claim 39, wherein said step of deriving saidtime interval is carried out by calculating a difference between saidsecond and third time values, dividing the determined difference by saidfirst stretch ratio to derive said length in time of said firstsynchronized pulse, calculating a difference between said sixth andseventh time values, dividing this determined difference by said secondstretch ratio to derive said length in time of said second synchronizedpulse, and deriving said time interval between the received first, andsecond pulses by subtracting the length in time of said secondsynchronized pulse from the length in time of said first synchronizedpulse and adding thereto said time interval between the synchronizededges of said first and second synchronized pulses.
 41. The method ofclaim 36, wherein said first synchronizing step is carried out byproducing said first synchronized pulse having a leading edgecorresponding to the first received pulse and a trailing edgesynchronized with an edge of one pulse of said clock signal; and saidsecond synchronizing step is carried out by producing said secondsynchronized pulse having a leading edge corresponding to the secondreceived pulse and a trailing edge synchronized with an edge of anotherpulse of said clock signal.
 42. The method of claim 41, wherein saidstep of deriving said time interval is carried out by calculating adifference between said second and third time values to produce a firststretched time interval representing a time length of said firststretched signal; and calculating a difference between said sixth andseventh time values to produce a second stretched time intervalrepresenting a time length of said second stretched signal; said firstcalibration step is carried out by calculating a difference between saidfirst and second time values to produce a first unstretched timeinterval representing a length in time of said first synchronized pulsebefore stretching, and calibrating the first stretch ratio from saidfirst stretched time interval and said first unstretched time interval;and said second calibration step is carried out by calculating adifference between said fifth and sixth time values to produce a secondunstretched time interval representing a length in time of said secondsynchronized pulse before stretching, and calibrating the second stretchratio from said second stretched time interval and said secondunstretched time interval.
 43. The method of claim 41, wherein said stepof determining is carried out by counting said elapsed number of clockperiods between the synchronized edge of said one pulse and thesynchronized edge of said another pulse of said clock signal.
 44. Themethod of claim 41, further comprising the step of accurately measuringthe number of periods of said clock signal from said second, fourth,sixth and eighth time values and said predetermined known period. 45.The method of claim 34, wherein said first and second calibration stepscalibrate the respective first and second stretch ratios from timevalues of a plurality of time interval measurements of a plurality offirst and second pulses.
 46. The method of claim 34, wherein both saidconverting steps generate said time values in accordance with a clocksignal; said method further comprising the step of supplying said clocksignal as the received clock signal.
 47. The method of claim 34, whereinsaid step of generating a first pulse sequence is carried out by summingthe first synchronized pulse and the first synchronized compared outputpulse; and said step of generating a second pulse sequence is carriedout by summing the second synchronized pulse and the second synchronizedcompared output pulse.
 48. The method of claim 34, wherein said firstconverting step includes the steps of receiving a stop signal, andascertaining respective time differences between first, second, thirdand fourth edges of said first pulse sequence and said stop signal toproduce first, second, third and fourth time values; and said secondconverting step includes the step of ascertaining respective timedifferences between first, second, third and fourth edges of said secondpulse sequence and said stop signal to produce fifth, sixth, seventh andeighth time values.